1. Field of the Invention
The present invention generally relates to completion operations in a wellbore. More particularly, the invention relates to an apparatus for hanging a string of liner from an upper string of casing within a wellbore.
2. Description of the Related Art
In the drilling of oil and gas wells, a wellbore is formed using a drill bit that is urged downwardly at a lower end of a drill string. After drilling a predetermined depth, the drill string and bit are removed and the wellbore is lined with a string of casing. An annular area is thus formed between the string of casing and the formation. A cementing operation is then conducted in order to fill the annular area with cement. The combination of cement and casing strengthens the wellbore and facilitates the isolation of certain areas of the formation behind the casing for the production or injection of hydrocarbons or other fluids.
It is common to employ more than one string of casing in a wellbore. In this respect, a first string of casing is set in the wellbore when the well is drilled to a first designated depth. The first string of casing is hung from the surface, and then cement is circulated into the annulus behind the casing. The well is then drilled to a second designated depth, and a second string of casing is run into the wellbore. The second string is set at a depth such that the upper portion of the second string of casing overlaps with the lower portion of the upper string of casing. Any string of casing that does not extend back to the surface is referred to as a liner. The second string is then cemented into the wellbore as well. This process may be repeated using additional strings of casing of an ever-decreasing diameter until the wellbore has been formed to the desired total depth.
The process of hanging a liner off of a string of surface casing or other casing string typically involves the use of a liner hanger. In practice, the liner hanger is run into the wellbore above the liner string itself. A connection is made between the liner and the liner hanger, typically via a threaded connection. A setting sleeve, in turn, is affixed above the liner hanger. These tools are made up together at the surface, and are run into the hole at the lower end of a landing string, such as a string of drill pipe. A temporary connection is made between the landing string and the setting sleeve, typically through a float nut. Additional tools may be employed with the running tool, including a slick joint and a wiper plug, depending upon the nature of the completion operation.
Several types of liner hangers are known in the art. In some instances, a mechanical liner hanger is used. A mechanical liner hanger is set typically through the use of rotational and axial motion imparted by rotating and moving the liner string up and/or down. Mechanical liner hangers are most often employed in connection with shallow and non-deviated wells. However, mechanical liner hangers are impractical for deeper wells and for wells which are deviated due to the difficulty in imparting the needed rotation and axial movement.
In the case of deeper wells and highly deviated wells, hydraulic liner hangers are more commonly employed. In order to set a hydraulic liner hanger, a ball is dropped into the wellbore and landed on a seat. The seat is positioned either in the running tool string, on a wiper plug or, in some instances, at a landing collar. Other types of seats are also known. Fluid is then injected into the wellbore under pressure in order to actuate the hydraulic liner hanger.
In known hydraulic liner hangers, fluid under pressure is injected through an inner mandrel of the liner hanger. Fluid passes through one or more ports and into a small annular area defined between the mandrel and a surrounding tubular body called a cylinder. Seals are placed within the annular area above and below the ports in order to confine fluid pressure. The cylinder is configured in such a manner that fluid pressure creates an upward force on the inner surface area of the cylinder between the seals, causing the cylinder to be urged upwardly.
FIG. 1 depicts a partial cross-sectional view of a prior art hydraulic liner hanger 10. Visible in this view is the inner mandrel 12 of the hanger 10, and the surrounding cylinder body 14. Above the cylinder 14 is a plurality of radially spaced-apart slip members 18. Each slip 18 has a base 16 that is connected to the cylinder 14. In this way, upward movement of the cylinder 14 will in turn drive the respective slips 18 upward.
The slips 18 are disposed upon outwardly angled surface areas called cones 20. The slips 18 are designed to ride upward upon the cones 20 upon activation of the cylinder 14 through hydraulic pressure. In this respect, hydraulic pressure forces fluid through ports 25 in the mandrel 12. Fluid is maintained under pressure within the cylinder 14 between upper 24 and lower 26 seals. Because of the configuration of the inner cylinder 14 surface, the injected fluid applies an upward force on the cylinder 14.
The cylinder 14 is releasably connected to the mandrel 12 by frangible member(s) 28. Typically, the frangible members 28 are shear screws. Upon a designated axial force caused by fluid acting upon the cylinder 14, the frangible member(s) 28 are broken, thereby releasing the cylinder 14. The cylinder 14 then moves upwardly along the outer surface of the liner hanger 10, forcing the slips 18 to ride upwardly and outwardly along the respective cones 20.
It can be seen in FIG. 1 that each slip 18 includes a set of teeth. These teeth are typically referred to as “wickers.” The wickers provide frictional engagement between the liner hanger 10 and the inner surface of the upper string of casing (not shown in FIG. 1). The liner, in turn, is threadedly connected to the bottom sub 22 of the liner hanger 10.
There are disadvantages associated with the use of known hydraulic liner hangers. First, it is evident that the ports 25 and seals 24, 26 between the cylinder 14 and the inner mandrel 12 of the liner hanger 10 are potential leak paths. In this respect, the seals 24, 26 and the surrounding cylinder body 14 are exposed to wellbore pressure and fluids during the life of the well. High downhole temperatures place great demands on the elastomer seals typically used on the cylinder 14. Failure of the seals 24 or 26 results in costly remedial work to repair the leak.
Associated with this problem is the inherent structural considerations for the cylinder 14. Hydraulic cylinders 14 are in contact with the wellbore fluids and are thus considered flow-wetted parts. The cylinder 14 is typically constructed of the same material as the liner 22 it is being used with in order to insure compatibility with the fluid. This adds to the cost of the typical liner hanger construction. Further, the high downhole pressures induce high burst and collapse loads on the hydraulic cylinder 14 along with additional stresses on the seals 24, 26 used. Thus, the required cylinder thickness can force compromises in the mandrel 12 thickness that reduces pressure and load capacities. In this respect, there is a limited amount of space between the bore of the inner mandrel 12 and the surrounding ID of the casing string. Increased thickness of the cylinder body 14 means less thickness available for the mandrel 12.
Hydraulic liner hangers 10 typically have a reduced annular bypass area due to the external hydraulic cylinder 14 used for setting them. The reduction of bypass area increases the surge pressures placed on the formation during run-in. Further, the reduced bypass area restricts the space for annular flow during cementing operations.
Finally, as noted, hydraulic liner hangers 10 typically employ frangible members 28 such as shear screws or rupture discs to prevent premature movement of the hydraulic cylinder 14 during run-in. The frangible member 28 is designed to retain the cylinder 14 in place until a specific internal pressure has been reached. However, if this pressure is prematurely exceeded due to a surge in downhole pressure, the slips could prematurely be released, causing the liner hanger 10 to set improperly within the wellbore. In addition, there is the potential that slip 18 deployment could take place where one or more slip members 18 encounter debris downhole. This again could cause premature setting of a hydraulic liner hanger 10. Hydraulic liner hangers 10 are typically not considered re-settable. If the hydraulic liner hanger 10 is prematurely activated, the liner 22 will likely not be able to run to the desired setting depth, causing additional drilling and additional length of liner to be used.
As can be seen, there is a need for an improved hydraulic set liner hanger. In this respect, there is a need for a hydraulic set liner hanger which eliminates the use of a cylinder body. Still further, there is a need for a hydraulic set liner hanger which does not employ ports through the wall of the liner hanger body, or seals which could become a source of leaks. There is yet a further need for a hydraulic set liner hanger which can be more easily unset in the event of premature actuation during run-in. Further, a liner hanger that has the above desired features and can be run below a compression set liner top packer. Further still, there is a need for an improved liner hanger which is simpler and more reliable than known hydraulic and mechanical liner hangers.